Stain blocking coating compositions (i.e., primers) are applied to the surfaces of substrates, including uncoated surfaces or previously coated substrates. These coatings promote adhesion and also serve as a barrier coating to underlying polar or non-polar staining agents. Substrates frequently contain soluble or mobile staining agents. Given the nature of water-based coatings, the staining agents often leach from the substrate into and/or through the coating, causing surface discoloration of the coating. For example, tannins contained in woods such as redwood, cedar, elm, merbau and mahogany can leach from the substrate into the coating, causing tannin staining, which appears as discoloration on the surface of the coating. The visual appearance of localized stains or discoloration also can be manifested from extractives of previously coated substrates due to substrate exposure to water or humidity spots. In addition, salts contained in cementitious substrates often cause efflorescence, which is a staining caused by the migration of the salt from the substrate to the paint coating, where it appears as white deposits. Staining of the substrate, and of coatings previously applied to the substrate, can also be caused by sources external to the substrate. For example, cigarette smoke causes nicotine staining, which discolors light colored coatings; inks from pens cause marker stains on the substrate. Each of these types of staining is highly undesirable in coatings.